While sizes of electronic devices are becoming smaller and their speeds are increasing, the raising density of circuits develops the very large scale integrated circuits. The metal wires in chips also become narrower, which results in a higher resistance and a larger capacitance. However, the signal transmission speed of traditional metal connection structure is far behind that of current devices. With the improvement of devices this kind of signal transmission delay will occur more and more frequently. Accordingly, the current best choice is to replace the traditional aluminum wire with the copper wire. However, it is quite troublesome that the copper interconnected chip in the wire bonding process is very difficult for bonding, since it is so easy to oxidize in high temperature that the copper oxide film is soft and lacks the protection ability. This is also an unsolved problem for many electronic packaging factories.
The wire bonding process is a key point for signal transmission and power supply. Also, it is one of the most vulnerable parts in the packaging process. Therefore, the wire bonding process plays an important role in the electronic packaging process. The bonding pad made of the copper chip is different from the traditional aluminum bonding pad in the wire bonding process. Their material properties and oxides are totally different. The oxide layer of aluminum is a film of very good protection ability. However, the oxide layer of copper is soft and unable to form a delicate protection film and its thickness will increase along with the raising of time and temperature.
Because of the aforementioned difference of copper pad, during wire bonding, not only is the thermosonic vibration effect reduced, but the friction power of the thermosonic vibration is hard to be transmitted to the surface of the copper pad, which makes it unable to successfully connect to the copper pad, and, therefore reduces the yield rate of the bonding pad. During the wire bonding process of the bare copper chip, the key point for the yield rate of the bonding pad is to prevent the copper chip from quickly oxidizing due to the influence of the bonding interface temperature.
In order to prevent the copper chip from quickly oxidizing due to the influence of the bonding interface temperature, the surface of chip is spread with Argon while performing the thermosonic wire bonding of bare copper chip. Except for the protection of inert gas, the pre-heat temperature could also be decreased to reduce the oxidization of the copper pad. Sputtering a barrier layer on the surface of the copper chip, is another way. So far, the titanium metal layer is used as a barrier for preventing the copper chip from oxidization. The present invention utilizes the high temperature feather of copper materials and provides a method to improve wire bonding process for copper connection. The oxide film of the copper chip is generated naturally in air circumstance with controllable temperature and humidity. The thickness of the oxide film is controlled and becomes a barrier to prevent oxidization. This method is cost effective and can increase the bonding yield rate.
It can successfully bond the metal wire on the copper interconnected chip with good bonding pad strength by using the copper oxide film as barrier layer. The present invention provides a method of thermosonic wire bonding process for copper interconnected chip by using the copper oxide film as barrier layer.